The invention generally relates to the Fluid Catalytic Cracking (FCC) process and more particularly to an apparatus and method for processing feed streams having very different compositions or boiling ranges in the same FCC unit.
In fluid catalytic cracking processes, it is often necessary to process different feed streams that have very different properties or boiling point ranges in the same FCC unit. These streams can be straight run or cracked materials from other conversion units, or recycled materials from the same FCC unit. One of the streams is generally the main feed while others are supplemental feeds intended to maximize production of a certain product from the FCC unit.
The various feed streams may require very different cracking conditions due to very different molecular size/structure. As a result, a number of modifications to the conventional FCC process have been developed in which these streams are fed at different locations in the riser reactor. In general, the lower boiling or lower molecular weight materials require more severe conditions to crack, while higher boiling materials require less severe conditions. Materials rich in aromatics are difficult to crack and form increased quantities of coke, which reduces the effectiveness of the catalyst. These considerations need to be taken into account in determining the best way to process the different streams.
U.S. Pat. No. 4,051,013, issued Sep. 27, 1977, is directed to a fluid catalytic cracking process for simultaneously cracking a gas oil feed and upgrading a gasoline-range feed to produce high quality motor fuel. According to this patent, the lower boiling gasoline-range feed is contacted with freshly regenerated catalyst in a portion of the riser reaction zone that is relatively upstream from the portion of the riser reaction zone in which the higher boiling gas oil feed is injected. However the lighter gasoline feed is injected at a single point that does not provide uniform and thorough contact of the catalyst and the feed.
U.S. Pat. No. 4,892,643, issued Jan. 9, 1990, discloses a catalytic cracking operation using a single riser reactor in which two different types of cracking catalysts are employed. In this process, heavy hydrocarbon feed is introduced to the riser reactor upstream from the lighter feed. Cracking of the heavy feed produces a significant quantity of naphtha, which is then combined with a downstream naphtha feed.
U.S. Pat. No. 5,846,403, issued Dec. 8, 1998, discloses a method of improving the yield of light olefins in a FCC process while simultaneously increasing the octane rating of gasoline produced in the process. According to this method, a light catalytic naphtha feed and steam are injected upstream of the conventional FCC feed injection point. However the lighter feed is injected at a single point of injection. This method does not provide uniform and thorough contact between the catalyst and the light feed, and, as a result, conversion and yield of the desired products are not maximized. Also, in this process the heavier feed is mixed with conventional FCC feed, i.e. gas oil, and is injected in the riser through the same feed injectors as the main feed. This design does not provide optimum conditions for heavier feed to vaporize and undergo the desirable catalytic cracking reactions.
Therefore, it would be useful to develop a method for processing different feeds in a single riser reactor of a FCC unit wherein the location as well as the method of injection are designed so that the different feed streams can uniformly and thoroughly contact the high activity catalyst at the catalyst temperature that is best suited for-maximum catalytic conversion of these individual streams, and the contact time between the catalyst and the different feeds is optimized so that production of the desirable products is maximized.